The primary objective in this 26 week study is to evaluate the efficacy of a new investigational antidiabetic drug, BRL 49653C, in reducing hyperglycemia when added to the therapy of NIDDM patients who are inadequately controlled on a maintenance dose (2.5g/day) of metformin. The primary endpoint will be the reduction from baseline in HBA1c after 26 weeks of treatment in BRL 49653C/metformin combination groups (BRL 49653C 4 mg or 8 mg qd plus metformin 2.5g/day) in comparison with the metformin monotherapy group (BRL 49653C placebo plus metformin 2.5g/day). Secondary objectives include defining the clinical safety and tolerability of the monotherapy and combination groups. Also the metformin monotherapy group will be compared with the BRL 49653C metformin combination groups in terms of their efficacy in reducing fasting plasma glucose (FPG), fructosamine, C-peptide, immunoreactive insulin and serum lipids including total cholesterol, HDL-cholesterol, LDL cholesterol, VLDL cholesterol, triglycerides and free fatty acids at week 26.